1. Field of the Invention
The present invention relates to temperature control for a nozzle and a barrel of an injection molding machine.
2. Description of the Prior Art
In starting injection molding operation in an injection molding machine, the respective temperatures of individual zones of a nozzle and a barrel must be raised to target preset temperatures. According to this conventional heat-up method, the preset temperatures are ordered as target values without regard to the heat-up characteristics of the zones of the nozzle and the barrel, and PID (proportional integral differential) control is executed to attain the target preset temperatures.
A zone of the nozzle portion with high watt density can be quickly heated to its target preset temperature. However, zones near the root of the barrel are heated up slowly and take time before their temperatures reach their respective target preset temperatures. Thus, the heat-up times of the zones of the nozzle and the barrel are subject to variation. Accordingly, a molten resin stays for various times in the individual zones before a screw is allowed to rotate after heat-up operation. In consequence, the molten resin stays long in the zone of the nozzle portion that can be quickly heated up, so that it is subject to problems of deterioration, burning, carbonization, etc.
In order to avoid these problems, a novel control method is disclosed in the Japanese Patent Application Laid-Open No. 5-77302. With this method, the start of heating is delayed more for zones with smaller thermal capacities so that the points of time for the attainment of preset temperatures for individual zones are substantially coincident. According to this control method, however, the temperature of the nozzle portion that has a smaller thermal capacity starts to rise later than those of the zones of the barrel do. Thus, the resin that stays in the barrel is decomposed, gasified, and accumulated in the barrel, possibly resulting in explosion.